In order to measure the temperature of a plasma, which is a high-speed transient phenomenon, the method has been considered of calculating the temperature by measuring the light emitted by the plasma. Conventionally, the method has been considered of finding the temperature by selectively simultaneously detecting the intensity of two atomic spectral lines that are emitted by the plasma and calculating the temperature from the ratio of their intensities. However, this method is subject to the problem that, as well as the atomic spectral lines, the plasma emission spectrum contains a continuous optical component, so when this continuous optical component is added in, the measurement accuracy is lowered. An example of such a method is described in Takeuchi et al. “Temperature and Metal Vapor Near the Cathode in Copper Breaking Arcs According to Spectroscopic Measurement”, IEEE Transactions on Plasma Science, Vol. 28, No. 3, pp. 991-999, 2000 (hereinafter referred to as Non-Patent Reference).